1. Field of the Invention
The present invention relates to a method of producing an SSB signal, to an SSB pulse modulator and to a transmitter including the SSB pulse modulator.
2. Description of the Related Art
An edge position modulation technique is disclosed in an article "Modulation Methods Related to Sine-Wave Crossings" by Konrad Piwnicki in IEEE Transactions on Communications, VOL, COM-31, No. 4, April 1983 pages 503 to 508. In this article an equation for the binary signal u(t) which represents the edge position modulated wave is developed where ##EQU1##
where
r(t) and s(t) are real signals, PA1 .omega..sub.c is the carrier frequency, and PA1 T.sub.n is the Chebyshev polynomial of the first kind of degree n.
FIG. 2 of this article illustrates a simplified scheme for generating the signal u(t) by employing two voltage comparators whose outputs are coupled to respective inputs of an EX-OR gate. The inputs to one of the two comparators are r(t) and cos .omega..sub.c t and those to the other of the two comparators are s(t) and sin .omega..sub.c t. The EX-OR gate yields a modulated wave with a d.c. component which can be eliminated by a capacitor.
In the above expression (1) when n=1 then ##EQU2## and this represents a quadrature modulated signal on a carrier of frequency .omega..sub.c with modulating signals s(t) and r(t). When n=2 then ##EQU3## and this represents a carrier of frequency 2.omega..sub.c amplitude modulated by s.sup.2 (t)+r.sup.2 (t).
Expression (2) may be regarded as a general form of a narrow-band modulated signal. The type of modulation is determined by the relation between the signals r(t) and s(t). If r(t)=s(t), where s(t) is the Hilbert transform of s(t), the modulated signal will be single sideband with suppressed carrier (SSB-SC) based on the lower sideband. If r(t)=-s(t), the modulated signal will be SSB-SC based on the upper sideband. A disadvantage of producing SSB-SC in this manner is that the output contains a high level of even harmonics of the carrier at low modulation depths. Of these even harmonics the levels of the odd numbered multiples, that is n.multidot.2.omega..sub.c, where n is an odd integer, are particularly high. Removal of these even harmonics can be achieved by particularly stringent output filtering. However this makes the modulator operation more suited to narrowband which is unsuitable for a signal, such as speech, having a wide dynamic range.
Another known SSB modulator uses polar inputs as opposed to Cartesian inputs to give a three level harmonic-free output. However the base band signal processing required is very complex.